Patent Documents 1 and 2 listed below disclose hollow poppet valves formed with an internal cavity in the valve head and the stem of the valve, with the space charged with a coolant (such as metallic sodium, which has a melting point of about 98° C.) having a higher heat conductivity than the valve material, together with an inner gas.
The internal cavity extends from inside the valve head into the stem adapted to contain a large amount of coolant for enhanced heat transfer property of the valve. (The property enhanced by the coolant will be hereinafter referred to as heat reduction effect.)
An engine is heated during an operation, and, if the temperature of the combustion chamber of the engine is excessively, knocking may take place, which lowers the fuel efficiency and hence the power of the engine. In order to lower the temperature of the combustion chamber, there has been proposed use of a coolant charged in an internal cavity formed in a valve together with an inner gas so as to positively enhance the heat transfer effect of the valve.